1. Field of the Invention
The present invention relates to methods and apparatus for depositing a silicon layer on a transparent conducting oxide (TCO) layer suitable for fabricating photovoltaic devices.
2. Description of the Background Art
Photovoltaic (PV) devices or solar cells are devices which convert sunlight into direct current (DC) electrical power. PV or solar cells typically have one or more p-i-n junctions. Each junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region. When the p-i-n junction of the PV cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through a PV effect. PV solar cells generate a specific amount of electric power and cells are tiled into modules sized to deliver the desired amount of system power. PV modules are created by connecting a number of PV solar cells and are then joined into panels with specific frames and connectors.
Typically, a PV solar cell includes a photoelectric conversion unit and a transparent conductive oxide (TCO) film. The transparent conductive oxide (TCO) film is disposed as a front electrode on the bottom of the PV solar cell in contact with a glass substrate and/or as a back surface electrode on the top of the PV solar cell. The transparent conductive oxide (TCO) layer is a conductive layer that provides high electricity collection and photoelectric conversion efficiency for the solar cells. The photoelectric conversion unit includes a p-type silicon layer, a n-type silicon layer and an intrinsic type (i-type) silicon layer sandwiched between the p-type and n-type silicon layers. Several types of silicon films including microcrystalline silicon film (μc-Si), amorphous silicon film (a-Si), polycrystalline silicon film (poly-Si) and the like may be utilized to form the p-type, n-type and i-type layers of the photoelectric conversion unit. Typically, the silicon films of the photoelectric conversion unit are deposited by a plasma enhanced chemical vapor deposition (PECVD) process. One problem with the formation of current thin film solar cells is that haze, discolor, or other similar types of defects may form on the TCO layer during deposition of materials thereover.
Therefore, there is a need for an improved method and apparatus for depositing silicon layer on a TCO layer.